1. Field of the Invention
The present invention relates to a stable cap assembly and a cylindrical rechargeable battery capable of increasing a contact area between a safety vent and a gasket by integrating a dual-gasket structure to thereby increase a sealing effect, increasing a short circuit reactivity of a current interrupting member by freely changing the gasket structure according to the diameter of the current interrupting member to thereby improve stability, simplify components, and reduce a production unit cost, and a cylindrical rechargeable battery including the same.
2. Description of the Related Art
As demand for developing a technique for mobile devices is increasing, demand for rechargeable batteries as an energy source is rapidly on the rise, and among them, a lithium rechargeable battery having a high energy density and discharge voltage has been actively studied and commercialized so as to be used.
A rechargeable battery is classified into a cylindrical battery and an angular battery including an electrode assembly installed in a cylindrical or angular metal can, respectively, and a pouch type battery including an electrode assembly installed in a pouch type case of an aluminum laminate sheet. Among them, the cylindrical battery has the advantage in that it has a relatively great capacity and is structurally stable.
Also, the electrode assembly installed in a battery case is a generating element which has a stacked structure including a positive electrode/separation film/negative electrode and can be charged or discharged. The electrode assembly is classified into a jelly-roll type electrode assembly formed by winding the positive and negative electrodes, long sheet type electrodes coated with an active material, and the separation film interposed therebetween, and a stack type electrode assembly formed by sequentially stacking a plurality of positive electrodes and negative electrodes with a separation film interposed therebetween. The jelly-roll type electrode assembly has the advantages in that it can be easily fabricated and has a high energy density per weight.
FIG. 1 is a schematic vertical sectional perspective view of a general cylindrical rechargeable battery.
As shown in FIG. 1, the cylindrical rechargeable battery 100 is fabricated such that a jelly-roll type (winding type) electrode assembly 120 is received in a cylindrical case 130, electrolyte is injected into the cylindrical case 130, and a top cap 140 including an electrode terminal (e.g., a positive electrode terminal (not shown)) is then coupled to an open upper end of the case 130.
The electrode assembly 120 has a structure in which a positive electrode 121, a negative electrode 122 and a separation film 123 are sequentially stacked and then wound in a circular form, and a cylindrical central pin 150 is insertedly positioned in a core (i.e., a central part of the jelly-roll). The central pin 150 is generally made of a metal material in order to provide a certain strength and has a hollow cylindrical structure formed by bending a plate material in a circular form. The central pin 150 serves to fix and support the electrode assembly and serves as a passage for discharging a gas generated due to an internal reaction during charging or discharging or operation therethrough. Meanwhile, the lithium rechargeable battery has shortcomings that it is instable. For example, when the battery is over-charged by more than about 4.5 V, an active material of the positive electrode is decomposed, a lithium metal is grown as dendrite from the negative electrode, and the electrolyte is decomposed. In this process, heat is generated to accelerate a plurality of sub-reactions such as the decomposition reaction, to cause an ignition and explosion of the battery.
Thus, in order to solve such a problem, as shown in FIG. 2, the general cylindrical rechargeable battery includes a current interruptive device (CID) 50 mounted in a space between an electrode assembly (not shown) and a top cap 10 in order to interrupt current and resolve a withstanding voltage when the battery operates abnormally.
The top cap 10 forms a protruded positive electrode terminal including an exhaust hole. A positive temperature coefficient (PTC) element 20 is formed under the top cap 10. When internal temperature of the battery goes up, battery resistance of the PTC element 20 greatly increases to interrupt current. A safety belt 30 is formed to be protruded downward in a normal state. When the pressure within the battery increases, the safety vent 30 is protruded to be ruptured to exhaust the gas. The CID 50 is configured such that one upper end portion is coupled to the safety vent 30 and one lower end portion is connected to a positive electrode of the electrode assembly. Portions of edges of the top cap 10, the PTC element 20, and the safety vent 30 are covered by a first gasket 60a, and the CID 50 is covered by a second gasket 60b. 
Under a normal operational condition, the positive electrode of the electrode assembly is electrically connected with the top cap 10 by way of a lead, the CID 50, the safety vent 30, and the PTC element 20.
Meanwhile, when a gas is generated from the electrode assembly due to a cause such as an over-charge, or the like, to increase an internal pressure, the safety vent 30 is protruded upward as its shape is reversed, so as to be separated from the CID 50, interrupting the current. As a result, over-charging is prevented and thus stability is secured. Nonetheless, if the internal pressure keeps increasing, the safety vent 30 is ruptured and pressurizing gas is exhausted through the exhaust hole of the top cap 10 by way of the ruptured portion, thus preventing explosion of the battery. The stability and safety of the battery can be secured while the processes are sequentially performed.
The first gasket 60a, which fixedly packs the top cap 10, the PTC element 20, and the safety vent 30, is generally made of a resin material such as polypropylene that can be easily deformed. The second gasket 60b, which supports the CID 50, is made of the same material as that of the first gasket 60a. 
However, as noted in FIG. 2, because the safety vent 30 is positioned at the lowermost end supported by the first gasket 60a and the CID 50 positioned at a lower side of the safety vent 30 is supported by the second gasket 60b, it happens that a portion of the safety vent 30 is not supported by the first gasket 60a, so sealing is likely to be not good.
When the sealing between the gasket and the safety vent is not good, moisture can be possibly infiltrate or electrolyte can be possibly leaked. The occurrence of such an electrolyte leakage may cause the electrolyte, an inflammable material, to be leaked out to be in contact with oxygen in the air to cause ignition, and moisture infiltrating into the battery may cause a decomposition reaction of the electrolyte to cause ignition or explosion.
Also, because the internal gas of the battery is partially leaked out, internal pressure required for operating the CID cannot be maintained. In detail, the operational process of the CID absolutely depends upon the amount of generated gas in the electrode assembly, so if the amount of generated gas is not sufficient or when the generated gas fails to increase to a certain level of amount within a short time, CID short-circuit is belated, so the continuous electrical connection of the electrode assembly would cause a thermal runaway phenomenon. The thermal runaway is generated or accelerated when the battery is placed in the continuous electrical connection state, increasing the likelihood that the battery is ignited or exploded, so there is a severe problem of stability. In addition, the recent incidents of ignitions of notebook computers stress the significance of stability and safety of batteries.
Therefore, development of a battery having a noble structure, which can solve the problems of the related art battery structure in which the imperfect sealing between the cap assembly and the gasket covering the cap assembly causes the electrolyte leakage and the inappropriate discharge of gas, is highly required.